I think the graph show's better aging by catchers simply because it's not expected to be a great offensive position. 27-year-old Jose Molina is already a pretty poor hitter, there's not much room for decline for 37-year-old Jose Molina. Or Miguel Olivo, Gerald Laird, Drew Butera, Fancisco Cervelli, John Buck, Dioner Navarro etc. It's just a position where low offensive output is accepted, and the lower the peak output, the lower the amount of statistical decline later. Teams just don't tolerate this lack of production from other positions except for shortstop to a degre.